1. Field of the Invention
The present invention relates generally to photomultiplier detectors and, more particularly, to a protection device and method for use therewith. The invention is particularly suited for protecting a photomultiplier detector against damage which can result from the application of excessive incident light flux while maintaining the operation of the detector at a safe level until the excessive incident light flux is reduced or removed.
2. Description of the Prior Art
Photomultiplier detectors are used in numerous applications where low light levels are sensed or detected. In such detectors, light energy is directed onto a photo-emissive cathode which emits electrons in proportion to the incident light energy. The emitted electrons are directed at a first of a series of dynodes that emits an increased number of electrons through a process known as secondary emission. The increased number of secondary emission electrons from each dynode is cascaded onto the next dynode within the series, producing an output current through an anode positioned to receive the electron emission from the last dynode. The amplification resulting from the secondary emission at each dynode can result in extremely sensitive photomultiplier detectors having gains or ratios of anode currents to cathode currents as high as 10.sup.8 or more.
Each dynode within the dynode series is connected to a source of relatively high voltage, the voltage increasing from the first to the last dynode in the series. Generally, a high voltage power supply serves as a high voltage source for a voltage divider network of fixed resistors that in turn supplies the various voltages required by the dynode series. The output current flowing from the photomultiplier detector is proportional both to the incident light energy falling upon the photo-emissive cathode and also to the voltage applied to the voltage divider network and in turn to the dynodes. The voltages applied to the dynodes from the voltage divider network are sometimes referred to hereinafter collectively as "dynode voltage". Thus for a given constant incident light flux or intensity, the output current flowing from the detector anode can be varied by varying the dynode voltage (that is, by varying the high voltage applied to the voltage divider network). Since the sensitivity of the photomultiplier detector can be expressed as the output current flow for a given constant incident light flux, it is seen that the detector sensitivity is varied by varying the dynode voltage.
A problem associated with using photomultiplier detectors is protecting a detector from combinations of incident light flux and dynode voltage which can produce a destructively high current flow within the detector. Such a situation can occur where the dynode voltage and thus the detector sensitivity is initially set for a first relatively low incident light flux. However, if the incident light flux is increased, the current multiplying effect within the photomultiplier device can raise the current flowing therethrough to a level which can damage or destroy the detector. Hence, it is necessary to protect the photomultiplier detector from combinations of incident light and dynode voltage which can result in an excessive current flow through the detector.
The problem of photomultiplier detector protection arises in the field of spectrophotometry where photomultiplier detectors are widely used in spectrophotometers to sense the light passing through a sample undergoing spectral analysis.
Several situations occur where the photomultiplier detector can be exposed to light intensities which can result in damage to the detector. Generally, spectrophotometers have a sample compartment that receives a sample for analysis. When placing the sample into the sample compartment, the ambient room light can flood the compartment and can enter the optical path leading to the photomultiplier detector. Ambient light levels in such an instance can be considerably above the light level applied to the detector during normal operation. Also, changes in light wavelength can result in substantial variations in incident light intensity because of the wavelength-dependent differences in spectral energy content of light emanating from the source within the spectrophotometer and because of non-uniformities in the dispersion of light by the optical elements within the spectrophotometer's optical path. Variations in photomultiplier detector spectral sensitivity for changing incident light wavelength can also lead to detector damage. Moreover, the photomultiplier detector can be exposed to high light intensities during servicing of the spectrophotometer instrument as, for example, when the optical path is opened to room ambient light. In each of these instances, it is possible for the photomultiplier detector to receive sufficient incident light flux which, along with the applied dynode high voltage, can result in damage to the detector.
Generally, two types of spectrophotometers are known. The first type is a double-beam spectrophotometer while the second type is a single-beam spectrophotometer.
In a dual-beam spectrophotometer, light from a light source is rapidly alternated between a sample beam path and a reference beam path. A sample material and a reference material are placed in the respective paths and the sample and reference beams are then multiplexed to form a combined single beam that is applied to a photomultiplier detector. The output current from the photomultiplier detector is demodulated to provide reference and sample signals corresponding to the light intensities in the respective reference and samp1e paths. The reference signal is applied to a dynode voltage control circuit which adjusts the dynode voltage to maintain the reference signal at a predetermined level related to a predetermined output current from the detector. The predetermined output current is selected to be well within the normal output current range of the detector.
If the reference signal varies from the predetermined level, as may normally result from changes in the light source intensity or drift in the photomultiplier detector sensitivity, the dynode voltage control circuit adjusts the dynode voltage in a direction and by an amount necessary to correct for the difference between the reference signal and the predetermined level. The dynode voltage control circuit continually operates in this manner to provide a relatively stable reference signal. As is well known in the art, the sample signal is compared with the reference signal to determine, for example, the transmittance of the sample material as compared to the reference material.
The reference signal adjustment process just described can also serve to protect the photomultiplier detector from over-current damage. For example, if stray ambient light should fall on the detector or if the intensity of the light source should vary considerably, the dynode voltage control circuit produces a corresponding change in the dynode voltage and a reduction in the photomultiplier detector sensitivity. The continual adjustment of dynode voltage thus maintains the detector output current within the normal output current range and substantially eliminates damage to the detector. In this way, the dynode voltage control circuit provides inherent protection of the photomultiplier detector in a double-beam spectrophotometer.
However, such inherent protection is not present in a single-beam spectrophotometer instrument. In such an instrument, the sample and reference materials are measured at different times in the same optical path. The reference material is first placed into the single-beam optical path and the dynode voltage is adjusted so that the detector output current is equal to a predetermined reference level within the normal output current range of the detector. Once the adjustment is completed, the dynode voltage is held constant. The reference material is removed from the single-beam optical path and is replaced with the sample material whereupon sample measurements are made. As is known in the art, the sample measurements are compared to the reference level to determine characteristics of the sample such as transmittance. If the intensity or flux of the light incident upon the detector during the sample measurement period increases substantially, the constant dynode voltage applied during this time can cause a damaging over-current condition in the detector. Thus, it is desirable to provide protection for the detector in a single-beam spectrophotometer so that wide variations in incident light flux will not lead to damage or destruction of the detector.
One way known for protecting the photomultiplier detector in such instruments is to provide a mechanical shutter arrangement which closes the light path from the sample compartment to the photomultiplier detector when the compartment is opened. Although this can be a satisfactory solution to the problem during normal operation of the spectrophotometer, the shutter arrangement increases the mechanical complexity of the spectrophotometer. Also, shutters can fail or become sluggish and cannot be provided to protect the detector against ambient light in all circumstances such as, for example, when the spectrophotometer is being serviced as described above.
Another form of photomultiplier detector protection known to applicants is included in a single-beam spectrophotometer instrument designated the model DU.RTM.-8, manufactured and sold by the assignee of the present application. In the DU-8 instrument, a comparing circuit monitors the output of the photomultiplier detector during sample measurement period and compares the output to a predetermined limit indicating that the output current is approaching a level above which damage would result. If the output reaches the limit because of excessive incident light flux for the constant dynode voltage, the detector dynode voltage is removed. With such an arrangement, however, it was not possible to determine when the excessive incident light flux is removed from the detector because detector operation ceases with the removal of the dynode voltage. Consequently, once the cause of the excessive light flux condition is corrected, it is necessary to restart the spectrophotometer operational cycle, a relatively time consuming and inconvenient process particularly where a number of sample measurements are to be taken.
Thus, there is a need for a photomultiplier detector protection circuit which not only senses the presence of an excessive light flux condition in order to protect the photomultiplier detector, but which also is able to sense when the excessive incident light condition is terminated so that operation of the spectrophotometer can continue without further interruption.